1. Field of the Invention
The invention relates to a method for etching, and more particularly to a method for etching with high etch selectivity.
2. Description of the Related Art
As device size of the integrated circuits decreases, the width of the contact hole and the opening becomes narrower, and the aspect ratio of the contact increases correspondingly. FIG. 1 is a cross sectional view of contact formed by a conventional etching process. In the prior art of an etching process, C.sub.x H.sub.y F.sub.z usually serves as an etching gas, to be pumped into the chamber to etch the oxide layer 102 on the substrate 100. Other gases further contain He/O.sub.2, hydrogen (H.sub.2), carbon oxygen (CO), etc. A contact hole 106 is next defined by forming a photoresist layer 104 on the oxide layer 12. However, due to the presence of hydrogen atoms in C.sub.x H.sub.y F.sub.z, it is difficult to control the C/F ratio in the reactive gas and polymerization is hence hard to control, too. Therefore, either it is hard to form a enough polymer thin film on the surface of the photoresist 104 and the contact hole 106, or the contact hole 106 is clogged by the polymer thin film. The etching process of contact can't be continuously performed. Oxygen in the He/O.sub.2 is useful to reduce the polymerization reaction in the formation of a polymer thin film on the surface of the photoresist 104 and the sidewalls of the contact hole 106. Hydrogen induces the formation of HF and results in higher C/F ratio in the chamber environment which increases the formation of polymer. But when oxygen and hydrogen are mixed together, the process becomes much more dangerous. Obviously, the safety of the process becomes more and more important.